1. Field of the Invention
The present invention relates to an electromagnetic wave suppression sheet including an electromagnetic wave suppression material used for a countermeasure against an unnecessary radiation generated from an electronic apparatus, to a device including an electromagnetic wave suppression sheet, and to an electronic apparatus equipped with an electromagnetic wave suppression sheet. In particular, the present invention is suitable for electromagnetic wave suppression in a GHz band.
2. Description of the Related Art
Along with the rapid popularization of recent telecommunication equipment, electromagnetic waves in a GHz band for a mobile phone (1.5, 2.0 GHz), a PHS (1.9 GHz), a wireless LAN (2.45 GHz, 5.0 GHz), or the like are rapidly increasingly used.
Further, there are an increasing number of apparatuses whose LSI drive frequencies exceed GHz, and their harmonics have higher-frequency components.
As described above, the number of potential electromagnetic wave generation sources, the number of interference-affected objects, and varieties thereof are being increased at an exponential rate. Therefore, the risk of causing an electromagnetic interference (EMI) in the GHz band is being astronomically increased.
To cope with a problem of the electromagnetic interference, it is necessary for individual apparatuses to have a sufficient immunity against an electromagnetic wave from outside (improvement of immunity) without emitting an unnecessary electromagnetic wave that hinders normal operations of other apparatuses (suppression of emission).
This is called electromagnetic compatibility (EMC). Various standards are defined in order for an electronic apparatus to establish the electromagnetic compatibility in an electromagnetic environment.
These days, as a method of overcoming a problem of the electromagnetic interference, an electromagnetic wave suppression sheet (electromagnetic wave absorption sheet) is being used. A general electromagnetic wave suppression sheet has a composite structure of a resin and magnetic material particles or conductive material particles, for example (see, for example, Japanese Patent Application Laid-open No. 2003-174279 (hereinafter, referred to as Patent Document 1)).
Here, a mechanism of conversion to heat energy is classified into three types of a conductive loss, a dielectric loss, and a magnetic loss. At this time, an electromagnetic wave absorption energy P (W/m3) per unit volume is expressed as the following expression (1)P=½σ|E|2+πf∈″|E|2+πfμ″|H|2  (1)where E represents an electric field, H represents a magnetic field, f represents a frequency of the electro magnetic wave, σ represents an electric conductivity, ∈ represents a complex electric conductivity (∈=∈′−j∈″), and μ represents a complex magnetic permeability (μ=ν′−jμ″).
In the expression (1), the first term expresses the electric conductivity loss, the second term expresses the dielectric loss, and the third term expresses the magnetic loss.
The electromagnetic wave suppression sheet used for the countermeasure for a part in which a large current change occurs is generally made of a magnetic material.
To suppress and absorb the magnetic components of the electromagnetic wave, the electromagnetic wave suppression sheet made of the magnetic material is designed so that an imaginary part μ″ of the magnetic permeability of the third term that expresses the magnetic loss in the above expression (1) is high.
The electromagnetic wave suppression sheet as described above is bonded on an LSI package, a printed circuit board, an FPC (flexible printed circuit), a harness, a metallic component, or the like, and implemented.
In addition, to increase an antenna efficiency for RFID, the electromagnetic wave suppression sheet is used, thereby suppressing the interference between an antenna and a metallic part.